This invention relates generally to radio frequency signal transmission and reception, and more particularly the invention relates to frequency synthesis circuitry used in radio transmitters and receivers.
The radio transmission of electrical signals involves modulation of a propagated carrier signal by a lower frequency signal containing the intelligence to be transmitted. Because of the crowded use of the radio frequency spectrum, certain frequency ranges are designated for particular users. For example, citizens band (CB) radio has been assigned forty transmission frequencies, or channels, in the range of 26.9 to 27.5 megahertz (MHz).
Typically, the frequency synthesizer in a CB radio is controlled by switch means whereby the various frequencies or channels are selected for transmission and reception of radio waves. However, the synthesizers are capable of generating frequencies other than the assigned frequencies, and by alterating the switch means a CB radio can be used for transmission on unassigned or illegal channels. This can create interference with other RF signals, particularly television signals.